A DC tachometer is usually in the form of a small DC generator which produces a potential proportional to speed. Normally the individual windings produce a sinusoidal signal as they pass through the magnetic field. The sinusoidal signals are then converted to a DC signal by means of the switching action of the commutator. Because of the sinusoidal shape of the individual windings and incremental switching action of the commutator, the output of the tachometer will include some undesirable ripple in the signal. In a typical DC tachometer the ripple content can be on the order of 6%. In higher quality tachometers which employ a larger number of commutator bars to reduce ripple content, the ripple content can be reduced to about 2%.
The concept of brushless DC motors is known. A suitable brushless tachometer design is disclosed in copending application entitled "Brushless Tachometer" by Phillip S. Coulon Jr. filed on even date herewith. The brushless DC tachometer is, in effect, an alternator coupled with solid state switching controlled in accordance with rotor position. In such devices, the ripple content of the output signal is a more severe problem since, due to practical considerations, the switching increments cannot be reduced significantly to reduce ripple content. Typically, a brushless DC tachometer would include a three-phase alternator and a six transistor switching bridge. With such an arrangement each switching increment would cover 60 electrical degrees of rotation and, if the windings produce sinusoidal signals, the tachometer output would include a large low frequency ripple component on the order of 13.5% of the output signal.
An object of this invention is to provide a brushless DC tachometer with a significantly reduced ripple content.
Another object is to provide a quality DC tachometer at reduced cost.
Still another object is to provide a low cost structure for a DC electromagnetic machine having a low ripple component.